Fierce fighting and clashes rocked an eastern district of Syria's capital Damascus on Friday, although the fragile truce in the country's five-year civil war appeared to be generally holding elsewhere for a fourth successive day.

"The Syrian army is blocking an attack by armed groups that tried to enter the capital's east via Jobar... leading to intense clashes and rocket fire," a government military source told AFP.

Rocket fire and shelling could be heard from the Jobar district, a rebel-held eastern suburb which has been a battleground for more than two years, and has seen almost the entire civilian population flee.

The Syrian Observatory for Human Rights, an independent monitoring group, also reported clashes and said more than 21 shells and rockets had hit Jobar. Two shells also hit the Bab al-Sharqi neighbourhood of Damascus but no one was hurt, the Observatory said.

Rami Abdel Rahman, who heads the Britain-based monitoring group, said that rebel fighters from both Faylaq al-Sham and Fatah al-Sham - formerly Al-Qaeda's Syrian affiliate al-Nusra Front - were present in Jobar. Fateh al-Sham has refused to sign up to the terms of the current truce.

Under the ceasefire deal, negotiated by Moscow and Washington, which came into force on Monday evening, fighting is to halt across the country except in areas where groups are present whom the US and Russia deem to be "terrorist".

Observers have noted that the deal will be particularly difficult to implement in areas where Fatah al-Sham has formed strong alliances with local rebels.

Al-Jazeera's correspondents on the ground in the western city of Homs, as well as in the north-western Idlib governorate, also reported bombings.

UN pleads for aid access

It had been hoped that the truce would allow much-needed humanitarian aid into besieged areas of Syria, but the still fragile security situation has so far made this difficult.

The United Nations on Friday urged Syria's government to allow immediate aid deliveries to starving civilians.

In a sign of renewed tension Moscow, who backs Bashar al-Assad's government forces, accused Washington of failing to meet its obligations to restrain its proxy fighters under their deal.

Russia's ambassador to the UN, Vitaly Churkin, said it wanted a Security Council resolution endorsing the deal. "We are working on it," he told reporters in New York, adding that he thought it should be adopted when the Security Council meets on Syria next Wednesday.

The UN said trucks loaded with aid had spent a second day stuck in a buffer zone between Turkey and Syria but added that it was hopeful the vital supplies could be delivered to besieged rebel-held districts of Aleppo city on Friday. The ceasefire deal calls for the demilitarisation of the key Castello Road into the city.

Russia said on Thursday that Syrian armed forces were "fulfilling their obligations and have started a gradual withdrawal" from the route, but rebel groups did not appear to be carrying out a simultaneous pullback as agreed.